Titanium alloy heat treatment process, and part thus obtained

ABSTRACT

Ti 5-5-5-3 titanium alloy heat treatment process having, in weighted percentages, the following composition:
         between 4.4 and 5.7% aluminum,   between 4.0 and 5.5% vanadium,   between 0.30 and 0.50% iron,   between 4.0 and 5.5% molybdenum,   between 2.5 and 3.5% chromium,   between 0.08 and 0.18% oxygen,   0.10% traces of carbon,   0.05% traces of nitrogen,   0.30% traces of zirconium,   0.15% traces of silicon,
 
the residual percentage being titanium and impurities, characterised in that the heat treatment of said alloy is carried out by:
   heating to a first stage of between 810 and 840′° C. and below the β-transus temperature of the alloy;   maintaining at the first stage for one to three hours;   cooling to a second stage of between 760° C. and 800° C. without intermediate reheating;   maintaining at the second stage for two to five hours;   cooling to ambient temperature;   heating to a third stage of between 540° C. and 650° C.;   maintaining at the third stage for four to twenty hours, then cooling to ambient temperature.       

     Part obtained by this process.

The present invention relates to a Ti 5-5-5-3 titanium alloy (meaning 5%aluminum, 5% vanadium, 5% molybdenum, 3% chromium on a titanium base)and more particularly a heat treatment for said alloy, the object ofwhich is to improve the standard and uniformity of its mechanicalproperties.

The Ti 5-5-5-3 alloy is a quasi-beta type titanium alloy which atambient temperature, has two phases, an alpha phase (hereinafter “α”)and a beta phase (hereinafter “β”), and which has a β transition(hereinafter “β-transus”) between a domain where the α and β phasescoexist and the pure β phase domain. The temperature at which theβ-transus is found varies between 840° C. and 860° C. depending on thecomposition of the Ti 5-5-5-3 alloy. The Ti 5-5-5-3 alloy has both lowdensity and high mechanical strength. This is why it is highly prized inapplications in the aeronautical field, for example to produce landinggear parts and structural parts. However, this alloy is very sensitiveto microstructural flaws. Parts made of Ti 5-5-5-3 are usually obtainedafter thermomechanical transformation steps followed by heat treatmentsteps.

The thermomechanical transformation steps are carried out in the betaphase domain, in other words at temperatures which are higher than theβ-transus temperature of the alloy and at which the beta phase grainsform the matrix of the alloy, then in the alpha-beta phase domain, inother words at temperatures which are lower than the β-transustemperature of the alloy.

The half-finished products obtained after the thermomechanicaltransformation steps have, at ambient temperature, a microstructurecomprising primary alpha phase in the form of globular particles andelongated particles, secondary alpha phase in the form of lamellarparticles, and beta phase. The primary alpha phase represents 10 to 30%of the structure. In the rest of this description, when a percentage ofthe structure represented by a given phase is discussed, it should beunderstood that, as is conventional, this percentage is measured byoptical micrograph image analysis: the extent of the area occupied bysaid phase on the micrograph is measured by comparison with a referencegrid.

After the thermomechanical transformation steps, the parts made of Ti5-5-5-3 alloy are subjected to conventional heat treatments to obtainthe desired mechanical properties.

However, after these heat treatments a large dispersion in themechanical properties of parts made of the Ti 5-5-5-3 alloy persists,particularly with regard to the properties of ductility, toughness,tensile strength and resistance to fatigue, which remain veryanisotropic in the alloy.

A common heat treatment of the Ti 5-5-5-3 alloy consists of carrying outin succession:

-   -   a solution heat treatment at a temperature lower than the        6-transus temperature of the alloy, thus generally between        700° C. and 815° C., for two to four hours, followed by air        cooling to ambient temperature;    -   and an ageing heat treatment between 540° C. and 650° C. for        about eight hours, followed by air cooling to ambient        temperature.

The dispersion of the mechanical properties of parts made of Ti 5-5-5-3alloy obtained after conventional heat treatments is due toheterogeneity of the microstructure of the alloy, which itself is theresult of the initial texture of the alloy following thethermomechanical transformation steps. In particular, after aconventional heat treatment, the Ti 5-5-5-3 alloy has a heterogeneousalpha phase distribution within the microstructure. Moreover, after aconventional heat treatment, the alpha phase appears in the form ofparticles elongated in a dominant orientation resulting from the forgingor rolling direction during the last thermomechanical transformations.This dominant orientation of the alpha phase particles leads tomechanical properties which, measured in a direction parallel to that ofthe alpha particles, are acceptable, but which are very inadequate in adirection transverse to that of the alpha particles.

The object of the invention is to improve the standard and uniformity ofthe mechanical properties of a part made of Ti 5-5-5-3 alloy whileavoiding the above-mentioned drawbacks of the prior art.

Accordingly, the invention relates to a heat treatment process for theTi 5-5-5-3 titanium alloy, which has the following composition, inweight percent:

-   -   between 4.4 and 5.7% aluminum,    -   between 4.0 and 5.5% vanadium,    -   between 0.30 and 0.50% iron,    -   between 4.0 and 5.5% molybdenum,    -   between 2.5 and 3.5% chromium,    -   between 0.08 and 0.18% oxygen,    -   0.10% traces of carbon,    -   0.05% traces of nitrogen,    -   0.30% traces of zirconium,    -   0.15% traces of silicon,        the residual percentage being titanium and impurities resulting        from production.

The heat treatment process according to the invention comprises thefollowing successive steps:

-   -   the titanium alloy is heated to a first heat stage temperature        of between 810 and 840° C. and lower than the β-transus        temperature of the alloy;    -   the titanium alloy is maintained at the first stage temperature        for one to three hours;    -   the titanium alloy is cooled to a second stage temperature of        between 760° C. and 800° C. without intermediate reheating;    -   the titanium alloy is maintained at the second stage temperature        for two to five hours;    -   the titanium alloy is cooled to ambient temperature;    -   the titanium alloy is heated to a third stage temperature of        between 540° C. and 650° C.;    -   the titanium alloy is maintained at the third stage temperature        for four to twenty hours, then cooled again to ambient        temperature.

The above-mentioned heat treatment stages are carried out attemperatures below the β-transus temperature of the Ti 5-5-5-3 alloy.

As explained earlier, the microstructure of the alloy after thethermomechanical transformations (forging or rolling, for example) isheterogeneous. The first stage according to the invention allows themicrostructure of the alloy which has been affected by the precedingthermomechanical transformations to be homogenised. The first stagetemperature is slightly lower than the β-transus temperature of the Ti5-5-5-3 alloy, so as to solution treat as much alpha phase as possiblewithout however eliminating this phase which remains necessary to avoidan excessive increase in the size of the grain. In fact, without aminimum quantity of alpha phase, the beta phase grains would growuncontrollably leading to a significant reduction in the mechanicalproperties, particularly tensile strength. Preferably, the temperatureand duration of the first stage are determined so as to obtain aquantity of alpha phase of between 2 and 5% at the end of the firststage.

The second stage according to the invention is defined so as toprecipitate an equiaxial globular primary alpha phase. Because of thefirst stage in which the microstructure of the alloy was homogenised,the new alpha phase nuclei appear in a homogeneous distribution in themicrostructure of the alloy and their growth occurs equiaxially duringthe second stage to form globular primary alpha phase particles.

Thus, at the end of the second stage, the microstructure of the alloy ishomogeneous and the first two heat stages carried out according to theinvention have allowed a homogeneous globularisation of the primaryalpha phase within the microstructure and an adequate proportion of saidprimary alpha phase to be obtained.

Because of the heat treatment according to the invention, the Ti 5-5-5-3alloy has homogeneous and improved mechanical properties (ductility,toughness, tensile strength and resistance to fatigue). Morespecifically, the presence of homogeneously distributed globular primaryalpha phase markedly improves the ductility of the alloy.

The inventors have been able to demonstrate that the compromise betweenthe tensile strength and ductility of the alloy was optimal when thequantity of globular primary alpha phase at the end of the second stagewas between 10 and 15%. The temperature and duration of the second stageof the heat treatment according to the invention are thereforepreferably determined to obtain a quantity of globular primary alphaphase at the end of the second stage of between 10 and 15% in a betaphase matrix. Preferably, the temperature of the second stage is between770° C. and 790° C.

The first stage is preferably carried out at a temperature between theβ-transus temperature less 20° C. and the β-transus temperature less 30°C. and the second stage is carried out at a temperature of between 770and 790° C.

The first and second stages are preferably performed successively.

The cooling speed between the first stage and the second stage ispreferably between 1.5° C. and 5° C. per minute, and cooling at the endof the second stage is carried out to ambient temperature at a speed ofbetween 5° C. and 150° C. per minute.

The third stage is known as ageing as is standard practice for this typeof alloy.

The titanium alloy is maintained at the temperature of the third stagefor six to ten hours, preferably about eight hours.

The invention also relates to a part made of Ti 5-5-5-3 alloy,characterised in that it has been obtained from a half-finished productobtained by the above heat treatment process.

Other advantages and characteristics of the invention will emerge onreading the following description given as an example and with referenceto the accompanying drawings, in which:

FIG. 1 is a micrograph of the Ti 5-5-5-3 alloy that has undergoneconventional heat treatment before ageing;

FIG. 2 shows diagrammatically an example of the three heat treatmentstages according to the invention;

FIG. 3 shows a micrograph of the Ti 5-5-5-3 alloy that has undergone thefirst and second heat treatment stages according to the invention;

FIG. 4 shows a micrograph of the above alloy after it has undergone thethird heat treatment stage according to the invention.

The Ti 5-5-5-3 alloy heat treatment process according to the inventionapplies to parts that have, as is standard, been shaped following one ormore thermomechanical transformation steps performed in the beta phasedomain, followed by steps performed in the alpha-beta phase domain.These may be thermomechanical transformation steps through rolling,forging or extrusion.

The parts obtained after such thermomechanical transformation stepshave, at ambient temperature, a microstructure comprising primary alphaphase in the form of globular particles and elongated particles,secondary alpha phase in the form of lamellar particles, and beta phase.Following the thermomechanical transformation steps, the texture of thealloy is affected (orientation of the different alpha phasemorphologies), and the microstructure of the alloy is veryheterogeneous. In particular, the alpha phase particles are in the formof needles which are distributed in particular in the region of the betaphase grain boundaries. The alpha phase particles may be contiguous andform ribbons which have a detrimental effect on the toughness andresistance to fatigue and ductility of the alloy.

Heat treatments to improve the mechanical properties of the Ti 5-5-5-3alloy have been intensively studied. However, the so-called conventionaltreatments do not result in a homogeneous microstructure of the alloy,such that the mechanical properties are anisotropic across the alloy andinadequate to meet the stricter requirements demanded for certainapplications, such as landing gear parts.

In fact, as shown in the micrograph in FIG. 1, after conventional heattreatment and before ageing heat treatment, the alpha phase particles 1have heterogeneous sizes and distributions within the microstructure 2of the alloy. After conventional heat treatment, the alpha phase 1moreover is in the form of elongated particles, oriented in a dominantorientation resulting from the forging or rolling direction during thefinal thermomechanical transformation steps. This dominant orientationof the alpha phase particles does not allow isotropic mechanicalproperties to be obtained within the alloy.

One of the objects of the heat treatment according to the invention istherefore to homogenise the microstructure of the Ti 5-5-5-3 alloy.

The inventors have developed an optimised Ti 5-5-5-3 alloy heattreatment as shown diagrammatically in FIG. 2, comprising the followingsteps and stages:

-   -   heating 3 the titanium alloy to a first heat stage temperature,        of between 810 and 840° C., and slightly below the β-transus        temperature of the alloy;    -   maintaining 4 the titanium alloy at the first stage temperature        for one to three hours;    -   cooling 5 the titanium alloy to a second stage temperature of        between 760° C. and 800° C., preferably, as illustrated, carried        out without maintaining the alloy at an intermediate temperature        between those of the first and second stages as described.        Cooling the alloy from the first stage which would take the        alloy to a temperature below that of the second stage and would        therefore require reheating must be avoided;    -   maintaining 6 the titanium alloy at the second stage temperature        for two to five hours;    -   cooling 7 the titanium alloy to ambient temperature;    -   heating 8 the titanium alloy to the temperature of a third stage        9 of between 540° C. and 650° C.;    -   maintaining 9 the titanium alloy at the temperature of the third        stage 9 for four to twenty hours, followed by cooling 10 to        ambient temperature, said cooling normally being carried out        using air.

The first stage 4 situated between 810° C. and 840° C. and a littlelower than the β-transus temperature of the alloy, according to theinvention, allows the microstructure of the alloy affected by theprevious thermomechanical transformation steps to be homogenised, and asmuch alpha phase as possible to be solution treated, without howevercompletely eliminating said alpha phase. Preferably, the temperature andduration of the first stage 4 are determined to obtain a quantity ofalpha phase of between 2 and 5% at the end of the first stage 4. Aminimum content of 2% ensures that the beta phase grains do not growuncontrollably, which would have the consequence of considerablyreducing the mechanical characteristics of the alloy particularly thetensile mechanical properties. In addition, an alpha phase content ofless than 5% is preferable to allow good homogenisation of themicrostructure of the alloy, and in particular to break the alpha phaseribbons which formed following the thermomechanical treatments.

As explained earlier, the β-transus temperature varies depending on theexact composition of the Ti 5-5-5-3 alloy. To obtain the requiredquantity of alpha phase, the temperature of the first stage 4 isdetermined depending on the exact composition of the Ti 5-5-5-3 alloyand its β-transus temperature. To achieve the alpha phase quantitypreferably required, the first stage 4 is carried out at a temperaturebetween the β-transus temperature less 20° C. and the β-transustemperature less 30° C., regardless of the Ti 5-5-5-3 composition.

The duration of the first stage 4 is between one and three hours and isa function in particular of the geometry and bulk (diameter, thickness)of the part. The bulkier the part, the longer the stage lasts.

The second stage 6, between 760° C. and 800° C. according to theinvention, is defined to allow the precipitation of globular primaryalpha phase. Because of the first stage which allowed a homogeneousstructure of the alloy to be obtained, the new alpha phase nuclei appearin the course of the second stage 6, in a homogeneous distribution inthe beta matrix of the alloy, and the growth of the alpha nuclei occursequiaxially during the second stage 6 to form globular primary alphaphase particles 11, as shown in FIG. 3.

Thus at the end of the second stage 6, the microstructure of the alloyis homogeneous and the heat treatment according to the invention allows,moreover, a homogeneous globularisation of the primary alpha phase 11 tobe obtained within the microstructure (see the micrograph in FIG. 3).The presence of globular primary alpha phase 11 distributedhomogeneously in the microstructure 12 of the alloy improves theductility of the alloy. The double solution treatment by the first twostages of the invention homogenises the microstructure of the alloy andprepares it so that it responds more isotropically to the ageingtreatment of the third stage. Thus, after the entire heat treatmentaccording to the invention, the mechanical properties within the alloyare perfectly isotropic and improved compared with those conferred by aconventional heat treatment.

The inventors have been able to demonstrate that the compromise betweenthe tensile strength and ductility of the alloy was optimal when thequantity of globular primary alpha phase 11 at the end of the secondstage 6 was between 10 and 15%. Preferably the second stage temperatureis between 770° C. and 790° C. to obtain a quantity of globular primaryalpha phase of between 10 and 15% at the end of the second stage 6.

The duration of the second stage 6 is between two and five hours and isalso a function of the geometry and bulk (diameter, thickness) of thepart. The bulkier the part, the longer the stage lasts.

Typically, for a part of complex form made of Ti 5-5-5-3 titanium alloywith a composition of:

-   -   5.60% aluminum,    -   5.03% vanadium,    -   0.33% iron,    -   4.87% molybdenum,    -   2.97% chromium,    -   0.14% oxygen,    -   0.01% carbon,    -   0.006% nitrogen,    -   0.01% zirconium,    -   0.03% silicon,        the residual percentage being basically titanium, having        thicknesses of material of about 150 mm, the first stage is        carried out at a temperature of about 830° C. (the β-transus        temperature of the alloy being about 850° C.) and is maintained        at this temperature for about 2 hours and 30 minutes, and the        second stage is carried out, without having removed the part        from the furnace and without having reheated it to reach the        second stage temperature, at a temperature of about 775° C. and        is maintained at this temperature for about four hours. These        treatment conditions allow an alpha phase quantity of between 2        and 5% to be obtained at the end of the first stage 4, and a        quantity of globular primary alpha phase 11 of between 10 and        15% at the end of the second stage 6, distributed homogeneously        within a beta-type matrix 12. In the micrograph in FIG. 3,        obtained after the first two stages according to the invention,        it can be seen that the alpha phase particles 11, in black, are        globular in form, are of homogeneous sizes and have a homogenous        distribution within the alloy structure.

At the end of the first stage 4, cooling to ambient temperature or to atemperature lower than that of the second stage 6 would not be inaccordance with the invention. In fact, such cooling, which would haveto be followed by reheating to the temperature of the second stage 6,would lead to the formation of Widmanstätten-type alpha phase (slendersecondary alpha phase), at the expense of the formation, during thesecond stage 6, of a minimum quantity of equiaxial globular primaryalpha phase required to obtain good ductility characteristics of thealloy after heat treatment.

The cooling speed 5 between the first stage and the second stage ispreferably between 1.5° C. and 5° C. per minute and is, for example,carried out without removing the part from the treatment furnace. Thepart therefore cools progressively in a controlled manner inside thefurnace, the set temperature of which has been lowered progressively orimmediately until it reaches the temperature of the second stage 6.

A cooling speed of over 1.5° C./min is preferred to avoid a changeoccurring in the distribution of primary alpha phase during coolingspeeds that are too low which could be detrimental to obtaining goodmechanical properties. On the other hand, a cooling speed of more than5° C./min could lead to the precipitation of needle-type alpha phasewhich is detrimental to obtaining good mechanical properties such aselongation at rupture. In fact, an excess of needle-type alpha phase inthe structure of the material increases the risk of brittle fracture.

Cooling performed in the open air is not usually advisable, as its speedis difficult to control and, in many cases, the temperature of the partdrop too low, requiring reheating to the second stage temperature. Suchreheating must be avoided, for the reasons already stated, and coolinginside the furnace is an advantageous solution for implementing theinvention. Moreover, carrying out air cooling by removing the part fromthe furnace requires handling the part at high temperature, which isdifficult to perform.

The first 4 and second 6 stages are, preferably, carried outsuccessively.

What is meant by “successively” is that the move from the firsttreatment stage 4 to the second treatment stage 6 is achieved byprogressively reducing the temperature during cooling 5 to pass from thefirst stage 4 to the second stage 6, without maintaining an intermediatetemperature which would be lower or higher than that of the first stage4.

This successive performance of the two stages is preferred, thusseparating them by a progressive cooling to avoid any change occurringin the primary alpha phase distribution during an intermediate stage,which could be detrimental to obtaining good mechanical properties.

The cooling 7 following the second stage 6 is carried out to ambienttemperature at a speed preferably of between 5° C. and 150° C. perminute. This is for example air cooling carried out after having removedthe part from the treatment furnace.

It is preferable that the cooling speed following the second stageshould be less than 150° C. per minute to avoid too heterogeneous ahardening between the surface and the core of the part and avoid therisk of contraction cracking (superficial cracking) during cooling.

A speed of at least 5° C. per minute is preferable to anticipate ahomogeneous response to the subsequent tempering treatment during whichhardening precipitation occurs.

The third stage 9 is known as the ageing stage as is standard practicefor this type of alloy, the object of which is to harden the alloy byalpha phase precipitation.

The titanium alloy is maintained at the temperature of the third stage 9for six to ten hours, preferably about eight hours. The microstructureobtained after this third stage 9 is illustrated in FIG. 4.

At the end of the heat treatment according to the invention, themechanical properties of the Ti 5-5-5-3 alloy are isotropic, and havebeen improved compared with those of parts made of Ti 5-5-5-3 alloyobtained by conventional heat treatments. Because of the heat treatmentaccording to the invention, it has been possible in particular toimprove the tensile strength and ductility of parts made of Ti 5-5-5-3.On the parts tested, Rm values of more than 1290 MPa, elongation values“A” of more than 5% and reduction in area values “Z” of more than 15%have in fact been obtained.

As a comparison, after conventional treatments very dispersed RM valuesof between 1230 MPa and 1360 MPa are obtained on the same part. Theelongation values are also very dispersed, between 0.7 and 6.8%. Thetreatment according to the invention allows high and far less dispersedRm values of between 1260 and 1300 MPa, and similarly high and far lessdispersed elongations, of between 5 and 7.5%. Generally, the treatmentaccording to the invention guarantees a minimum Rm value of 1260 MPa anda minimum A value of 5%, whereas conventional treatments do notguarantee these minimum values.

The effects of the invention are particularly remarkable on bulky parts,in other words parts with a thickness or diameter of more than 100 mm.

Once the alloy has been treated according to the invention, finishingoperations continue as usual in the prior art to obtain the final part.

1. Ti 5-5-5-3 titanium alloy heat treatment process having, in weightedpercentages, the following composition: between 4.4 and 5.7% aluminum,between 4.0 and 5.5% vanadium, between 0.30 and 0.50% iron, between 4.0and 5.5% molybdenum, between 2.5 and 3.5% chromium, 0.08 to 0.18%oxygen, 0.10% traces of carbon, 0.05% traces of nitrogen, 0.30% tracesof zirconium, 0.15% traces of silicon, the residual percentage beingtitanium and impurities resulting from production, characterised in thata heat treatment of said alloy is carried out, comprising a plurality ofsteps and heat stages distributed in the following way: the titaniumalloy is heated to a first heat stage temperature of between 810 and840° C., lower than the β-transus temperature of the alloy; the titaniumalloy is maintained at the first stage temperature for one to threehours; the titanium alloy is cooled to a second stage temperature ofbetween 760° C. and 800° C. without intermediate reheating; the titaniumalloy is maintained at the second stage temperature for two to fivehours; the titanium alloy is cooled to ambient temperature; the titaniumalloy is heated to a third stage temperature of between 540° C. and 650°C.; the titanium alloy is maintained at the third stage temperature forfour to twenty hours, then cooled to ambient temperature.
 2. Processaccording to claim 1, characterised in that the temperatures anddurations of the first and second stages are determined to obtain analpha phase quantity of between 2 and 5% at the end of the first stageand a globular primary alpha phase quantity of between 10 and 15% at theend of the second stage.
 3. Process according to claim 1, characterisedin that the first stage is carried out at a temperature between theβ-transus temperature less 20° C. and the β-transus temperature less 30°C., and in that the second stage is carried out at a temperature ofbetween 770° C. and 790° C.
 4. Process according to claim 1characterised in that the first and second stages are carried outsuccessively.
 5. Process according to claim 1, characterised in that thecooling speed between the first stage and the second stage is between1.5° C. and 5° C. per minute and cooling at the end of the second stageis carried out to ambient temperature at a speed of between 5° C. and150° C. per minute.
 6. Process according to claim 1, characterised inthat the titanium alloy is maintained at the third stage temperature forsix to ten hours, preferably for about eight hours.
 7. Part made of Ti5-5-5-3 alloy, characterised in that it has been obtained from ahalf-finished product obtained by the heat treatment process accordingto claim 1.